Autoimmune disease arises when the immune system actively targets and destroys self-tissues, leading to a range of clinical syndromes. Multiple sclerosis (MS) is an autoimmune disease of the central nervous system and remains a major cause of disability in both young and older populations. It is generally believed that MS is mediated by immune responses against myelin antigens, followed by neurological impairment including the destruction of the myelin sheath and axonal loss. Experimental autoimmune encephalomyelitis (EAE) is the most commonly used animal model for MS, and is induced by immunization with disease-causative self-antigens such as myelin oligodendrocyte glycoprotein (MOG). Autoimmune diseases including MS are currently incurable. Current strategies to treat autoimmune disease often rely on nonspecific immunosuppression. Despite achieving some success, immunosuppressive strategies have difficulty in achieving long-term disease relief, and often lead to the development of immune deficiency with a consequence of a high rate of infections. The most logical way to treat autoimmune diseases including MS would be to induce immune tolerance to disease-causative-antigens whilst maintaining generalized immunocompetence by applying the same mechanisms that the immune system uses to maintain self- tolerance throughout life. To achieve this tolerance, the thymus, the major organ implicated in self-tolerance induction, has to be sufficiently functional, and the self-antigen has to be persistently present in the thymus. However, it is well known that the thymus undergoes age-dependent involution, and its functions are seriously compromised in the adult. Thymic epithelial cells (TECs) are the major component of the thymic microenvironment for T cell development. TEC degeneration is the major cause of age-dependent thymic involution. We have reported that mouse embryonic stem cells (mESCs) can be induced in vitro to generate thymic epithelial progenitors (TEPs) that further develop into functional TECs in vivo. We have also shown that transplantation of mESC-TEPs expressing MOG (MOG/mTEPs) in mice induces MOG self-antigen-specific tolerance, prevents EAE development and reverses established EAE. Recently, we have developed an efficient protocol to induce the differentiation of human ESCs (hESCs) to develop into TEPs. In this application, we propose to: 1) investigate the mechanisms by which transplantation of MOG/mTEPs prevents and treat EAE, and 2) establish new humanized EAE models by transplantation of hESC-TEPs or TEPs from induced pluripotent stem cells (hiPSCs) of MS patients and then determine the ability of these cells expressing MOG or other myelin proteins to prevent and treat EAE. Our proposed studies have the potential to lead to a new and powerful approach for preventing MS and treating patients this disease. This approach could also be used in the treatment of other autoimmune diseases when the causative self-antigens are known. In addition, our studies will also provide new insights into the mechanisms for immune tolerance and MS pathogenesis.